There are about 6,000 lakes in Lithuania, covering 950 km², or 1.5% of the territory of Lithuania. The lakes are not evenly distributed; most are situated in the Baltic Highlands, which begin near the border with Poland on the southeast and extend northward along the border with Belarus to Latvia. About 1,200 are supported only by groundwater and neither receive nor distribute any surface water. However, many of the other lakes are interconnected by complex passages and rivulets. These lake systems are major tourist attractions in Aukštaitija National Park and are popular with kayakers. The lakes in and near the Molėtai district municipality are a weekend destination for many residents of Vilnius, who have built summer houses and villas in the area. Lake origins The great majority of Lithuania's lakes formed after the Wisconsin glacier retreated. The oldest date from about 13,000 years before the present. The varieties of glacial lakes include: Moraine-dammed lakes (Lithuanian: užtvenktiniai ežerai) formed when glacial moraines blocked glacier meltwater drainage. These lakes include Dysnai, Plateliai, and Vištytis Kettle lakes (Lithuanian: guoliniai or termokarstiniai ežerai) formed when a large piece of ice broke away from the edge of a retreating glacier, and was then buried under its sediments. After the piece melted, a small depression was left in the landscape that filled with water. These lakes tend to be small, round, and quite deep. Due to their small size most of them are unnamed. The best-known of these is Lake Druskonis. (Lithuanian: Ledo guolio ežerai) formed much like kettle lakes, but are much larger. The pieces of ice were not buried under sediment. These lakes are often irregularly shaped, with uneven bottoms. Examples include Dusia, Metelys, Obelija, and Kretuonas. (Lithuanian: Dubakloniai, latakiniai, or rininiai ežerai) formed when meltwater washed steep, narrow, and deep valleys. When the valleys filled with water afterwards, lakes such as Asveja, Tauragnas, Sartai, and Aisetas were formed. Residual lakes (Lithuanian: liekaniniai ežerai) are the remains of large lakes that formed immediately adjacent to melting ice caps. These lakes are large, shallow, and surrounded by wetlands and peat bogs. Examples include Rėkyva, Žuvintas, and Amalvas. Some lakes are of mixed origins, created when a dam was constructed and the resulting reservoir flooded one or more lakes of glacial origin. Examples include Drūkšiai, Didžiulis or Daugai, and Galvė. The lakes of non-glacial origins include: Oxbow lakes (Lithuanian: senvaginiai, salpiniai, or upiniai ežerai) are abundant.